1. Field of the Invention
The present invention relates generally to apparatus for processing a video signal to be recorded suitable for use with a video tape recorder. More particularly, this invention relates to an apparatus for processing a video signal to be recorded, by which a so-called level inversion phenomenon can be avoided.
2. Description of the Prior Art
Conventionally, in a video tape recorder (VTR), in order to improve an S/N (signal-to-noise) ratio, a signal component with a high frequency band in a video signal is pre-emphasized and the pre-emphasized video signal is frequency modulated and recorded at the recording side whereas the pre-emphasized video signal is de-emphasized and returned to the original video signal at the reproducing side. If the signal level of the high frequency component of the pre-emphasized video signal is increased too much, an undesired influence such as an over-modulation and the like will be exerted upon the video signal upon FM-modulation. Therefore, at the recording side of the VTR, in order that the level of the signal component of the video signal does not exceed a threshold level, the above video signal is clipped and then frequency modulated.
If a video signal 1 shown, for example, in FIG. 1A is passed through a pre-emphasis circuit (not shown), the high frequency component of the video signal 1 is differentiated to produce a sharp overshoot portion 4 and a sharp undershoot portion 5 at a leading edge portion 2 to the white level and at a trailing edge portion 3 to the dark level as shown in FIG. 1B, reference numerals 19 and 21 designate predetermined dark and white clipped levels, respectively.
If the pre-emphasized video signal is supplied, as it is, to an FM-modulator circuit of the next stage and the FM-modulated video signal is recorded on and reproduced from a magnetic tape, the FM-modulated video signal is in a so-called over-modulated condition so that in extreme cases, a so-called level inversion phenomenon will occur.
The reason that this level inversion phenomenon, for example, white peak level inversion phenomenon occurs in this example, will be described with reference to FIGS. 2A to 2C.
As illustrated in FIG. 2A, an FM-modulated video signal la rises up abruptly from the dark level of the original video signal to its white level at a leading edge portion 2. Thus, as shown in FIG. 2B, a magnetized pattern 7 at the portion 2 where the frequency is changed from low to high becomes such that a magnetized pattern b is cancelled out by a magnetized pattern a. That is, a reproduced waveform 8 of the video signal becomes such that, as shown in FIG. 2C, an amplitude 9 of the reproduced waveform 8 at which the magnetized pattern a is changed to the magnetized pattern b is reduced and the reproduced waveform 8 does not cross the zero level. As a result, an FM demodulator circuit at the reproducing side of the VTR determines that the frequency of the video signal at this point is considerably low, so that the luminance signal becomes the dark level.
Such inversion phenomenon also occurs in the dark level peak portion due to the following reason.
That is, if the video signal is pre-emphasized, after the pre-emphasized video signal is FM-modulated, the frequency of, for example, 3.6 MHz to 4.8 MHz is expanded to the frequency of about 2 MHz to 6.5 MHz and then there is produced a ternary higher harmonic wave. Then, a phase relationship between the primary and ternary higher harmonic waves causes an abnormal zero cross so that the FM-demodulating circuit judges the dark level to be a white level.
Without such level inversion phenomenon, the S/N ratio of the video signal is deteriorated at the top portion of the overshoot portion 4 or the undershoot portion 5.
To remove the defect as described hereinabove, it is proposed in the prior art to provide a clipper or level slice circuit between the pre-emphasis circuit and the FM-modulator circuit to clip the overshoot portion 4 or the undershoot portion 5 at a level higher than a certain threshold level.
This previously-proposed method will be described with reference to FIG. 3. FIG. 3 illustrates an example of a known video signal recording apparatus.
Referring to FIG. 3, a video signal 1 is supplied to a video input terminal 10 and then is fed through a pre-emphasis circuit 12 and pre-emphasized therein. The pre-emphasized video signal therefrom is supplied to an FM-modulator circuit 13, in which it is frequency modulated. The FM video signal is supplied to a magnetic head 14 by which the FM video signal is recorded on a magnetic tape 15. A clipper or level slice circuit 18 is provided between the pre-emphasis circuit 12 and the FM-modulator circuit 13. The level slice circuit 18 uses, for example, an NPN transistor 16. In the level slice circuit 18, a signal line 20 for a video signal from the pre-emphasis circuit 12 is connected to an emitter e of the NPN transistor 16 of which the collector c is directly coupled to a reference voltage source +B of 12V. Further, the base b of the NPN transistor 16 is connected to an adjusting sliding contact 17 of a potentiometer 22 to perform a so-called white clipping operation in which the white level side of the video signal is clipped. A capacitor 11 is connected between the sliding contact 17 of the potentiometer 22 and the ground and each clip level is adjusted by the potentiometer 22 connected to the reference voltage source +B.
The known VTR in which the level slice circuit 18 using the NPN transistor 16 is provided in the recording system circuit as described above cannot overcome the following defect. That is, the signal component clipped by the level slice circuit 18 is dropped from the reproduced video signal returned to the original condition by the de-emphasis circuit at the reproducing side so that the reproducibility of the waveform is deteriorated by the above mentioned clipping. In other words, if a video signal 1 as shown in FIG. 4A is pre-emphasized at the recording side of the VTR and the overshoot portion 4 of the video signal 1 is sliced by a predetermined upper slice level 21 as shown in FIG. 4B, a waveform distortion 22a corresponding to the sliced signal component is produced in a reproduced video signal at the reproducing side as shown in FIG. 4C after the de-emphasis. Since the conventional level slice circuit 18 slices the video signal itself, if the slice level is increased, not only the waveform will be disto ted but also the sliced information will be lost completely. It is well known that the improvements of the waveform characteristic and of the inversion phenomenon are contrary to each other. So, it was very difficult to improve the waveform characteristic and to prevent the inversion phenomenon at the same time.
On the other hand, in order to make the frequency characteristic of the video signal flat in the recording/reproducing system of the known VTR, t is proposed to reduce a recording current (current of frequency modulated video signal) supplied to a magnetic head (rotary magnetic head) as the frequency: of the video signal is increased.
For this reason, if the frequency characteristic is applied to the recording current instead of providing the level slice circuit, the attention of the high frequency component of the FM-modulated video signal is decreased, so that the occurrence of the above inversion phenomenon can be avoided however, the frequency characteristic of the video signal can not be made flat in the recording/reproducing system of the VTR.
By the way, since the high frequency component near the frequency of about 1 MHz or above tends to be attenuated in electromagnetic conversion characteristic, if the recording current to be supplied to the magnetic head (rotary magnetic head) is reduced as the frequency becomes high, the attenuation of each of the frequency components is emphasized.